After advancing to Earthly Yuan Stage, the most vital step was to absorb Yin Energy, that existed between Heaven and Earth, into one’s body. After which, the Yin Energy will merge with the Yuan Power that lies in one’s body, greatly enhancing the attacking potential of one’s Yuan Power. Subsequently, when one advances to Heavenly Yuan Stage, one must absorb the Yang Energy, that exists between Heaven and Earth, into one’s body. Eventually, when the Yin and Yang Energy within one’s body synergizes, a Yuan Dan will be formed. This was eventually called Stealing Yin and Yang. When the two of them interacted with each, it was akin to water mixing with fire. They are completely incompatible.
Thus for the Earth Yuan and Heavenly Yuan stages, Yin and Yang Energy could be considered the most important components. However, within Heaven and Earth, Yin and Yang Energy can be classified into different grades and categories, with each possessing their own advantages and disadvantages. Most people would however, choose to absorb Yin Yang energy directly from Heaven and Earth because it was the most convenient method. After all, special Yin Yang Energy is not so easily chanced upon. Naturally though, this method would result in them being slightly weaker.
Of course, some wealthy people would rather choose to absorb special Yin Yang Energy. This way, they will end up distinctly stronger than a practitioner on the same Stage as them.
Besides, the higher the quality of Yin Yang Energy absorbed during the Earthly and Heavenly Yuan stages, the stronger the Yuan Dan formed will be!
Thus, some wealthier people will expend vast resources in order to find various kinds of special Yin Yang Energy to allow their Yuan Dan formed to be of a higher quality.